Heretofore, a liquid crystal display device has been used for mobile phones, PDAs, liquid crystal televisions, etc. A liquid crystal display device basically comprises a planar light-emitting device as a backlight, and a liquid crystal unit disposed on the light-emitting surface side of the planar light-emitting device.
A planar light-emitting device may be a direct type or an edge-lit type. A direct type light-emitting device requires light sources of about the same size as the light-emitting surface since light sources are disposed on the rear side on the opposite side from the light-emitting surface. An edge-lit type light-emitting device employs light sources smaller than the light-emitting surface since light sources are disposed at the edges in a direction at right angles to the light-emitting surface, and thus the edge-lit type light-emitting device is suitable for an increase in the size of the screen of the liquid crystal display device.
One example of the constitution of a light guide plate unit to be used for an edge-lit type planar light-emitting device is shown in FIG. 1. A light guide plate unit 10 shown in FIG. 1 is constituted by a light guide plate 20 to transmit light from a light source (not shown) disposed on a side surface to the interior by total reflection and to emit the light flatly, and a reflecting plate 30 to return light emitted from a light-reflecting surface (lower surface in the drawing) on the opposite side of the light guide plate 20 from a light-emitting surface (upper surface in the drawing) which mainly emits light, to the light guide plate 20 again. On the light-reflecting surface of the light guide plate 20, a light-scattering part 40 in a dot pattern is provided. The light-scattering part 40 scatters light totally reflected in the interior and emits the light from the light-emitting surface, and suppresses non-uniformity of the brightness on the light-emitting surface in a case where the light source is a point light source, non-uniformity of the brightness on the light-emitting surface due to a difference in the distance from the light source, etc.
As the light guide plate 20 in the light guide plate unit 10, a plate made of a transparent resin material such as an acrylic resin or a polycarbonate resin has been used in view of a high transmittance, and a relatively low cost and availability.
In a case where the size of a planar light-emitting device is increased along with an increase in the size of the screen of a liquid crystal display device, a higher output light source is used, whereby the amount of heat generated from the light source increases, and a light guide plate is required to have heat resistance. The above light guide plate made of a resin material has a low glass transition point (Tg) of from 80 to 100° C. in the case of an acrylic resin or from 145 to 150° C. in the case of a polycarbonate resin, and thereby has insufficient heat resistance. Further, an acrylic resin is problematic in that it has absorption in the vicinity of the wavelength of 780 nm.
As a light guide plate more excellent in the heat resistance than the above light guide plate made of a resin material, a light guide plate made of a glass material has been proposed (Patent Documents 1 and 2). The glass transition point (Tg) can be increased to a level of 530° C. depending upon the composition of the glass material.